This is a good question. Not only have I never met anyone in this sort of relationship, no one I've ever known has told me that they have ever known anyone in this sort of relationship. I'm wondering if anyone on CathInfo (other than poche, of course) actually knows anyone in this sort of relationship.
It seems to be an issue that is pure theory but does not actually exist in reality.
Approximately 36 years ago I met such a couple. We were intending to buy a property from them. We had to pass through their town and they gave us accommodation in their home, on our long journey to view the property.
One was a lapsed Catholic and the other was divorced (from somebody else). The had been living together in sin, but had "found Jesus" in one of those born-again halleluiah types religions. They took it very seriously when they were told that they either had to separate or to marry (the divorce was not a problem). So while they were waiting to be married, they continued to live together in the same house, but did not indulge in marital rights outside the married state.
They told us this was why they slept in separate rooms. Eventually they became missionaries in India.
Truth is stranger than fiction!